Polysiloxane fluids, particularly those with methyl functionality on the siloxane backbone, are known in the art to be unstable in the presence of oxygen at high temperatures. Oxygen, when present, is known to enter into the fluid by diffusion; therefore all contain a gradient of oxygen content and may gel preferentially at the surface. The inventor theorizes, but should not be held to the belief that the mechanism resulting in the instability is as follows:
It is believed that in the presence of oxygen the following reaction (I) occurs producing a silanol and formaldehyde. ##STR1## The silanol groups, thus formed, can further undergo a reaction (II) to produce water and high molecular weight polysiloxanes or gels. These reactions appear to be significantly inhibited when aryl groups, such as phenyl, are contained on the silicon in place of the aliphatic chains.
Another primary mechanism of thermal degradatons in siloxane fluids is that of "backbiting". Backbiting in siloxane fluids is illustrated in equation (III). ##STR2## In this process the molecular weight of the polymer decreases and under high temperature low molecular weight cyclics are vaporized.
Using zirconium and other metal compounds to stabilize organopolysiloxanes used in high temperature applications has been shown in the prior art. The inventor speculates that these compounds add stability by inhibiting the reaction described by I-III above. U.S. Pat. No. 4,122,109 to Halm shows a method of preparing methylpolysiloxanes with improved stability using Ti, Zr, or Hf compounds or their decomposition products. However these materials were tested and found useful only in non-oxidative, anhydrous environments. Kishimoto et al., U.S. Pat. No. 4,637,889 teaches a mixture of an organopolysiloxane, a zirconium-containing organopolysiloxane and a cerium-containing organopolysiloxane for use as coupler fluids. This invention is dependent on synergism of both the cerium and zirconium additives to provide the desired stability.
Zirconium compounds have also been shown in prior art to be useful in improving the stability of silicone rubbers. For example U.S. Pat. No. 2,658,882 to Maneri teaches a silicone rubber that resists deterioration of physical properties at 150 to 250 degrees Celsius by the addition of metallic zirconates, metallic fluorozirconates and zirconium silicates into the unvulcanized silicone rubber composition.
The use of metallocenes, especially ferrocene, to improve thermal stability in organopolysiloxanes is also well known in the art. For example, Awe et al., U.S. Pat. No. 3,002,989 shows a method for preparing siloxane fluids for use in high temperature applications by the addition of a ferrocene compound. This process requires a pre-oxidation step at temperatures from 450 to 600 degrees Fahrenheit. Brown et al., U.S. Pat. No. 3,745,129 shows the use of polymeric silylorganoferrocenes for improving the stability of organopolysiloxanes. However the process for making the polymeric silylorganoferrocenes is complex and solubility of the polymeric silylorganoferrocenes in the organopolysiloxane was not always readily achieved.
From the prior art, it was not obvious to use the specific metallocenes and organometallic compounds of this invention to stabilize phenylmethylsiloxanes in a thermal oxidative environment.
It is an object of this invention to provide phenylmethylsiloxane fluids which show improved thermal stability in oxygen and moisture containing environments.
It is a further object of this invention to provide a simplified method for making thermo-oxidatively stable phenylmethylsiloxanes using certain zirconium additives.
It is a further object of this invention to show the improved performance of the phenylmethylsiloxane fluids stabilized with zirconium in low surface area to volume applications.